CVE-2026-72316: dm era: fix NULL pointer dereference in metadata_open()

In the Linux kernel, the following vulnerability has been resolved:

dm era: fix NULL pointer dereference in metadataopen()

metadataopen() returns NULL when kzallocobj() fails, but the caller eractr() only checks ISERR(md). Since ISERR(NULL) returns false, the NULL pointer is treated as a valid result and later assigned to era->md, leading to a NULL pointer dereference when the metadata is accessed.

Fix this by returning ERRPTR(-ENOMEM) on allocation failure, consistent with dm-cache-metadata.c, dm-thin-metadata.c, and dm-clone-metadata.c which all use ERRPTR(-ENOMEM) for the same pattern.